A Twin Zygosity Test is a DNA test that definitively shows whether twins are identical or fraternal. During OB-GYN visits, the physician might be able to tell whether twins are identical or fraternal through ultrasound examination of the placenta. When the twins are born, pathological examination of the placenta can also be done to determine zgosity.


However, studies have shown that either method is not 100% accurate, and scientists recommend DNA testing to determine zygosity. There are also many cases in which medical records regarding zygosity have been lost, or doubt may have arisen because of the twins’ physical characteristics as the grow up.


In such cases, only a DNA test will be able to reveal the truth. A twin zygosity test compares the twins’ DNA profiles to see whether they match—an exact match proves that the twins are identical. The results of a twin zygosity test may be used to satisfy personal curiosity as well as to help solve health problems for the twins later down the road. For example, in the event that a twin needs an organ or tissue transplant donor, the identical twin is a perfect choice.


Identical vs. Fraternal Twins

Identical twins come from one fertilized egg, called a zygote. The zygote, which usually develops into one child, grows and splits early in development to form two embryos—identical twins. Because the twins come from one egg and one sperm, they have exactly the same DNA.

Fraternal twins, on the other hand, develop when there are two different eggs in the mother’s womb that are fertilized by two different sperms. Fraternal twins will not have exactly the same DNA, although like other siblings, they can be expected to share some of the DNA they inherit from both parents.   


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Twin Zygosity Test